1. Field of the Invention
This invention relates to a method of producing a thin film of polydiacetylene compounds. The invention particularly relates to a method of producing a thin film of polydiacetylene compounds with few defects and exhibiting excellent mechanial strength and more particularly to a method of producing such a thin film of polydiacetylene compounds by building up spread films on water surfece of diacetylene compounds on a substrate by Langmuir-Blodgett's technique or the like.
The invention can be applied for the formation of ultra-thin insulating films as well as in conjunction with ultra-thin film lithography. Specifically, it can be used for building up patterned thin films on a substrate of mercury cadmium telluride or cadmium telluride, thereby to obtain a high-precision patterned passivation film exhibiting excellent mechanical strength.
2. Description of the Prior Art
Diacetylene compounds having one or more conjugated groups in the molecule thereof can be polymerized by irradiation with ultraviolet rays or an electron beam, and it has been proposed to apply polydiacetylene compounds obtained in this way as resist materials in lithography processes, electrical insulating films in MIS (Metal Insulator Semiconductor) type junction devices and passivation films used following the formation of semiconductor circuit wiring.
Heretofore known methods of producing thin films of polydiacetylene compounds include Langmuir-Blodgett's technique, the horizontal lifting method and the rotational lifting method, wherein a monolayer or a built-up film is produced by dripping onto a water surface a diacetylene compound solution obtained by dissolving a diacetylene compound in a solvent such as chloroform, evaporating the solvent, and forming a monolayer by pressing the so-obtained spread film at a prescribed surface pressure while at the same time transferring it to a substrate. Where the so-formed whin film of diacetylene compounds is to be used for forming a resist film, the practice has been first to form a built-up film on the substrate in a dark environment and then to convert the diacetylene compound film into a polymer thin film by irradiation with patterned ultraviolet rays. (See, for example, Japanese Patent Unexamined Disclosure No. 58(1983)-111,029.)
On the other hand, where the thin film of diacetylene compounds is to be used for forming an ultra-thin insulating film, it has been the practice to complete the polymerization while the film is spread on the water surface and then transfer it to the substrate (see Japanese Patent Unexamined Publication No. 62(1987)-1701), or alternatively to build up a number of unpolymerized spread films on a substrate and thereafter polymerize them, in either case thereby obtaining a thin film of polydiacetylene compounds.
When a diacetylene compound having the skeletal structure shown in FIG. 2(A) is converted by irradiation with ultraviolet rays or an electron beam to a diacetylene polymer as shown in FIG. 2(B), a change occurs in the orientation angular of the diacetylene molecules.
This structural change is accompanied by a contraction of the diacetylene compound. As a result cracks occur in the polymer thin film and divide its surface into island-like regions called domains. These domains have been reported to measure from several micrometers to several millimeters (Gunter Lieser et al, Thin Solid Films, 68(1980) 77.about.90). The formation of these domains results in the presence of defects in the polydiacetylene compound thin film formed on the substrate so that regardless of whether the thin film is used as an ultra-thin insulating film or as an ultra-thin protection film for a resist material, it becomes an important matter from the practical standpoint to decrease domain formation.
As regards application as a protective film on a semiconductor, while good insulating films can be obtained on GaAs, InP, ZnS, ZnSe and SiC since these are amenable to ordianry sputtering, chemical vapor deposition and other such processes, it is not possible to obtain satisfactory insulating films on mercury cadmium telluride or cadmium telluride by these process since the heat they involve damages these semiconductor materials. There is thus a need for a film-forming method which does not cause damage to such semiconductor materials. Further, in use of these semiconductor materials for the production of semiconductor devices, there ordinarily arises the problem of the presence of native oxides and excess tellurium caused by etching on the surface of the semiconductor substrate, which tends to degrade device characteristics. Consideration must therefore also be given to the method of treating the substrate.